The present invention relates to a multiplexed electrical signal processor having a plurality of processing channels, for example a multiplexed multi-channel filter unit in which resistances are replaced by switch-controlled capacitors which form electrical charge-carrying components.
Numerous systems exist in which devices for analyzing the frequency spectrum of an electrical signal are used. This is the case in circuits which analyze speech signals. These circuits may, in particular, be employed in systems which are directly controlled by speech. It therefore is necessary that the apparatus should be capable of identifying spoken instructions given to it for controlling the different functions performed by this apparatus. However, the signal processor according to the invention is in no way limited to the application for analyzing speech signals.
As has already been indicated, the invention is concerned with a processor which makes use of switch-controlled capacitors as the equivalent of resistances. This technique is well known per se and has the advantage that it renders the integration of the circuit as a whole easier than it would be when using resistances. It will be helpful nevertheless to describe briefly the principle of operation of such switch-controlled capacitors. For this purpose, reference will be made to FIGS 1a and 1b.
FIG. 1a shows a capacitor C' connected between earth and the point which is common to two switches L.sub.1 and L.sub.2. The switches L.sub.1 and L.sub.2 are connected respectively to voltage sources V.sub.1 and V.sub.2. FIG. 1b shows the periodic logical control signals of the switches L.sub.1 and L.sub.2. The signals f.sub.1 and f.sub.2 are constituted by pulses of logical level 1 and period t'. The pulses applied to the control inputs of the switches L.sub.1 and L.sub.2 effect the closing of these switches. Thus, the pulse I.sub.1 of the signal f.sub.1, which closes the switch L.sub.1 effects the charging of the capacitor C' to the voltage V.sub.1 and the pulse I.sub.2 of the signal f.sub.2 effects the discharge of the capacitor C' to the voltage source V.sub.2. At this instant the signal f.sub.1 is at the level 0, so the switch L.sub.1 is open. On the average, during the interval t' which separates two successive pulses I.sub.1 or two successive pulses I.sub.2 the circuit acts as if a resistance R equal to t'/C' had been connected between the voltage sources. It can be seen, therefore, that this equivalent resistance depends both on the value of the capacitor C' and on the period t' of control of the switches L.sub.1 and L.sub.2.
Known multiplexed multi-channel filters use as the basic circuit an amplifier with a plurality of switched electrical charge-storing feedback capacitors connected in parallel and a plurality of charge-transferring capacitors, constituting the equivalent of filter resistances, which are connected to the input of the amplifier and are controlled by multiplexing and switching signals. Each charge-storing feedback capacitor defines a filter channel. The multiplexing signals control sequentially the charging and discharging of the charge-storing capacitor. The time constant associated with each filter channel depends on the relationship between the values of the charge-storing capacitor and of the charge-transferring capacitor corresponding to that channel and of the period of time elapsing between the two control pulses corresponding to one and the same storage capacitor.
FIG. 2 shows a time diagram of the multiplexing signals of a multichannel multiplexed filter according to the prior art. The multiplexing signals A.sub.1, A.sub.2, A.sub.3, A.sub.4, controlling the switches associated with the various different charge-storing capacitors all have the same period T and their pulses of logical level 1 (J.sub.1, J.sub.2, J.sub.3, J.sub.4) are phase-shifted one with respect to one another. Such a multi-channel multiplexed circuit has two major disadvantages. On the one hand, it leads to excessive current consumption by the operational amplifiers associated with the filter circuit and on the other hand it necessitates the use of capacitors which can have very high values. The first disadvantage is due to the fact that the current consumption of the operational amplifiers is directly proportional to their working frequency. FIG. 2 shows that, if there are n switching signals A.sub.1 to A.sub.4 (n=4), n control pulses will appear during the period T. If F=1/T is the frequency of the signal A.sub.1, the amplifier operates at a frequency n.times.F.
The other disadvantage is due to the fact that the time constant associated with each filter channel depends on the period T of the control signal of the channel in question and on the relationship of the capacitors corresponding respectively to the charge-transferring capacitor and the charge-storing capacitor of the filter channel in question. Since the period T is the same for all the channels, it is easy to understand that, for certain time constants, it will be necessary to use high value capacitors in order to obtain the desired relationship of the capacitors.
The object of the present invention is to provide a multiplexed signal processor which alleviates the disadvantages referred to above.